The invention relates to a high-frequency (HF) high-intensity discharge (HID) lamp assembly, more particularly to a hot-restrike ignition system for such lamp assembly
Discharge lamps operate with current limited by a ballast. There are various types of ballasts. At one end is the simple conventional ballast inductor used in neon lights at powerline frequency. At the other end are the electronic ballasts, in which semiconductors are used to control lamp current. The control elements in electronic ballasts operate at a much higher frequency than is the case with conventional ballasts, and the output frequency of electronic ballasts may be controlled independently from the current regulating stage of the ballast.
Electronic ballasts are commonly used for location-lighting in filming environments, where the current regulator operates at a high frequency, say 25 kHz, and the output frequency of the ballast is low, say 100 Hz, This is achieved by passing the regulated current through a low-frequency inverter. Such lamps typically operate at power levels anywhere between 200 watts and 18 kwatts. More information on the difference between conventional magnetic ballasts and electronic ballasts can be found in Chapter 5 of xe2x80x9cPhilips"" Medium Source High Intensity Discharge Lamps: Information for Luminaire and Ballast Manufacturersxe2x80x9d, published by Philips Lighting BV, Belgium, March 2000.
Some electronic ballasts drive the lamp of a lamp assembly at high frequency. This gives the advantage of a simple power stage because a low-frequency output inverter is not required. However, operation in this manner is generally limited to low-power lamps because of the problem of xe2x80x9cacoustic resonancexe2x80x9d (see Section 5.2 of the above Philips reference), In order to avoid acoustic resonances, the driving frequency must be above the highest resonant frequency of the lamp. It is lamp assemblies operating at these higher frequencies that are the subject of this invention.
FIG. 1, which is taken from Section 4.1 of the above Philips reference, illustrates the different phases between the moment of switching on the supply power and stable lamp operation for a HID lamp. A successful ignition operation proceeds through all of the voltage-versus-time phases illustrated in FIG. 1, while an unsuccessful ignition results when the process stops in one of those phases. The time scale in FIG. 1 is logarithmic so as to better illustrate the times involved.
The phases in FIG. 1 are: (a) ignition delay, (b) breakdown, (c) take-over, (d) glow, (e) glow-to-arc transition, (f) run-up (arc) phase, and (g) stable operation. The stable operation phase after ignition corresponds to that in which xe2x80x9csteady-state voltagexe2x80x9d and xe2x80x9csteady-state currentxe2x80x9d operate, as those terms are further used in this document.
Ignitor circuits presently in use with hot-strike-ignition high-intensity-discharge lamps are normally connected in series with the power circuitry that creates the steady-state current for the lamps. With a lamp assembly operating at the high frequency used with the subject invention, however, such a series connection is impractical due to the high heating and high impedance that would be added to the circuit. So the ignitor circuit of the invention is instead connected in parallel with the power circuit.
Reliable ignition of HID lamps is of crucial importance for location lighting in the film industry since filming cannot proceed without proper and sufficient lighting. Any resting delay has a significant cost implication. When HID lamps are hot, they become very difficult to ignite since the required ignition voltage becomes higher. This is the so-called xe2x80x9chot-restrikexe2x80x9d condition There is reference to such hot-restrike ignition capability in the patent literature. For instance, WIPO Patent Publication WO 97/43875 refers to a xe2x80x9chot restrike conditionxe2x80x9d with regard to a metal halide lamp. However, the voltage required for hot-restrike ignition of the lamp in that reference is considerably below that required by a metal halide lamp used for location lighting in the film industry, where lamps typically operate at a power level above one-half kilowatt,
It would therefore be an advantage to be able to hot-strike-ignite high-intensity discharge lamps of the type used for location lighting and which operate from a high-frequency ballast. It would be a further advantage to be able to ignite such a lamp quickly, avoiding costly down-time, while also reducing the size, weight and complexity of such a lamp as much as possible.
One aspect of the invention is a hot-restrike ignition system for a high-frequency high-intensity discharge lamp assembly. The assembly includes resonance means connectable across a lamp of the lamp assembly in parallel with a ballast circuit of the lamp assembly and which is temporarily energizable for producing a voltage sufficiently high for hot-restrike ignition of the lamp.
Preferably, the lamp assembly includes a ballast circuit for producing a steady-state alternating current for a lamp of the lamp assembly, and the resonance means is a resonant circuit means connectable across the lamp so as to extend in parallel with the ballast circuit, the resonant circuit means being temporarily energizable by energizing means.
The resonant circuit means may comprise one resonant circuit for producing an alternating voltage of higher voltage than a steady-state alternating voltage, and another resonant circuit for producing voltage spikes on the higher voltage. The one resonant circuit may comprise, in serial connection, a pair of coils and a capacitor means therebetween. The other resonant circuit may comprise, in serial connection, another coil, another capacitor means, and a spark gap means.
The pair of coils and the capacitor means may form a secondary circuit of a transformer means, with the other coil forming a primary winding of the transformer means.
A first connection switch may be positioned between one of the pair of coils and the capacitor means, closing of the first connection switch energizing the one resonant circuit.
The one resonant circuit may have a resonant frequency along with multiples and sub-multiples of that frequency, and that resonant frequency may approximate a natural open-circuit frequency of the ballast. The frequency of the steady-state alternating current may be in the range of between 300 kHz and 400 kHz. The spark gap means may discharge approximately every two milliseconds, each time creating a signal with a voltage spike followed by a declining amplitude and having a resonant frequency of approximately 15 MHz.
The one resonant circuit of the transformer means may produce an alternating voltage additive to the steady-state alternating voltage, a resulting total voltage amplitude of the one resonant circuit being at least twice that of the steady-state alternating voltage. The amplitude of the steady-state voltage may be approximately 150V, and the resulting total voltage amplitude at least 300V. Each voltage spike in the other resonant circuit may result in a corresponding voltage spike of approximately 25 kV amplitude in the one resonant circuit,
Energizing circuitry connected to the another resonant circuit may include a half-bridge rectifier, wherein one node of the rectifier is connected to a supply neutral through a second connection switch when the energizing circuitry is energized.
Another aspect of the invention is a hot-restrike ignition system for a high-frequency high-intensity discharge lamp assembly, the ignition system comprising a resonant circuit means and an energizing means. The resonant circuit means is in parallel with a ballast circuit that produces steady-state current in a lamp of the lamp assembly. The resonant circuit means comprises a transformer having a primary circuit and a secondary circuit. The secondary circuit comprises: a secondary winding of the transformer, the secondary winding having first and second portions with substantially equal number of turns; a secondary capacitor means positioned between the first and second portions of the secondary winding so as to be in serial connection therewith; and, a first connection switch positioned between the secondary capacitor means and one of the first and second portions of the secondary winding. The primary circuit comprises: a primary winding of the transformer, the primary winding being in serial connection with a primary capacitor means and a spark gap for producing a superimposed voltage wave-form on the secondary winding, the superimposed voltage waveform being of higher voltage and lower repetition rate than that produced by the secondary circuit; and, energizing means for temporarily energizing the primary and secondary circuits.
The primary winding of the transformer may be an output end of a circuit having as an input end a half-bridge rectifier, the rectifier being connected to a supply neutral through a second connection switch.
The first and second connection switches may be connected so as to open together and close together.
The invention is also a lamp assembly that includes the hot-restrike ignition system described above.
Preferred features of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: